AM broadcast stations are assigned operating signals which are spaced at intervals that limit the bandwidth of the transmitted signals. In the United States, AM radio broadcast stations are assigned operating frequencies which are spaced at 10 kHz intervals, and in some countries stations are at 9 kHz intervals. FCC rules now limit AM broadcast stations to a maximum modulating frequency of 15 kHz. Within the constraints imposed by broadcasting standards, relatively high fidelity reception of AM signals is possible in modern broadcast receivers. Such high fidelity reception requires ideal operating conditions including a strong station signal, lack of strong station signals of competing stations with immediately adjacent assigned frequencies, and lack of atmospheric or environmental noise at the receiver site.
The full IF bandwidth is usable in a receiver under the aforementioned ideal conditions. Since the aforementioned conditions for reception are not as a rule present, there usually is noise and annoying whistles in the audio signal. A variety of technical measures have been developed to cope with other than ideal reception conditions. Early AM receivers, for example, included manual controls that permitted selection of the IF bandwidth in two or more steps. More recently, notch filters have been inserted in the audio path to reduce the annoying whistle that sometimes results from the 10 kHz spacing requirement. To further improve reception, provisions have been added to automatically control the Q of notch filters in the audio path of receivers. These measures tend to eliminate adjacent channel carrier whistle and other station noise. It is recognized, however, that manual IF controls are not particularly useful and that merely reducing the Q of a notch filter does not completely eliminate adjacent channel interference.